1. Field of the Invention
This invention relates to a method of fabricating a light emitting device and thus-fabricated light emitting device.
2. Description of the Related Art
Light emitting device having the light emitting layer section thereof composed of an (AlxGa1-x)yIn1-yP alloy (where, 0≦x≦1, 0≦y≦1; simply referred to as AlGaInP alloy, or more simply as AlGaInP, hereinafter) can be realized as a high-luminance device over a wide wavelength range typically from green region to red region, by adopting a double heterostructure in which a thin AlGaInP active layer is sandwiched between an n-type AlGaInP cladding layer and a p-type AlGaInP cladding layer, both having a larger band gap. Current is supplied to the light emitting layer section through a metal electrode formed on the surface of the device. The metal electrode acts as a light interceptor, so that it is formed, for example, so as to cover only the center portion of a first main surface of the light emitting layer section, to thereby extract light from the peripheral region having no electrode formed thereon.
In this case, smaller area of the metal electrode is advantageous in terms of improving the light extraction efficiency, because it can ensure larger area for the light leakage region formed around the electrode. Conventional efforts have been made on increasing the energy of light extraction by effectively spreading current within the device through consideration on geometry of the electrode, but increase in the electrode area is inevitable anyhow in this case, having been fallen in a dilemma that a smaller light extraction area results in a limited energy of light extraction. Another problem resides in that the current is less likely to spread in the in-plane direction, because the dopant carrier concentration, and consequently the conductivity ratio, of the cladding layer is suppressed to a slightly lower level in order to optimize emissive recombination of carriers in the active layer. This results in concentration of the current into the region covered by the electrode, and consequently lowers the substantial energy of light extraction from the light leakage region. There has been adopted a method of forming, between the cladding layer and the electrode, a low-resistivity GaP light extraction layer having a dopant concentration larger than that of the cladding layer. The GaP light extraction layer increased in the thickness to a certain degree is not only successful in improving the in-plane current spreading effect, but also in increasing extractable energy of light from the side faces of the layer to thereby raise the light extraction efficiency. It is necessary for the light extraction layer to be formed using a material having a band gap energy larger than a light quantum energy of the beam of emitted light, for the purpose of efficient transmission of the beam of emitted light and raising the light extraction efficiency. In particular, GaP is widely used for composing the light extraction layer of AlGaInP-base light emitting device, by virtue of its large band gap energy and small absorption of the beam of emitted light.
Because a GaAs substrate used for growth of the light emitting layer section is a light-absorbing substrate (or opaque substrate), so that one known technique is such that the GaAs substrate is removed by lapping or etching after the growth of the light emitting layer section, and instead a GaP transparent substrate layer is formed by bonding of a single crystal substrate or by the vapor phase growth method. This configuration, having the opaque substrate on the second main surface side of the light emitting layer section replaced by the GaP transparent substrate layer, can extract light also from the side faces of the transparent substrate, and can make the light reflect on a reflective layer or an electrode on the second main surface side of the GaP transparent substrate layer, so as to allow extraction of the reflected light together with direct beam coming from the first main surface side, and can consequently improve the light extraction efficiency of the device as a whole. The GaP light extraction layer and GaP transparent substrate layer will generally and conceptually be referred to as “GaP transparent semiconductor layer”.
It has generally been considered that combination of half-dicing of a wafer and breaking based on cleavage can further simplify individualization of chips, if the side face areas of the GaP transparent semiconductor layer are agreed with the {110} surface which is a cleavage plane of GaP single crystal (allowing a degree of shift of 1° to 25°, both ends inclusive, away from the exact {110} direction, for the case where the off-angle is given as described in the above). Even in the process of full-dicing of the wafer for separation into chips, agreement of the dicing plane with the cleavage plane can suppress the load of the dicing to a low level, wherein also the chipping is less likely to occur. Aiming at full exhibition of the above-described advantages, it has been a fixed idea for III-V compound semiconductor devices having the zincblende structure, but not limited to the light emitting device within a scope of this invention, to adjust the direction of dicing to the <110> direction when they are manufactured by dicing wafers having the (100) main surface (also simply referred to as (100) wafer, hereinafter) as shown in FIG. 25. For example, Japanese Laid-Open Patent Publication “Tokkaihei” No. 8-115893 discloses a method of fabricating a light emitting device, involving dicing of a (100) wafer in parallel with the orientation flat, wherein the orientation flat of the (100) wafer is generally formed in parallel with the {110} surface, so that the dicing direction described in Japanese Laid-Open Patent Publication “Tokkaihei” No. 8-115893 is in the <110> direction.
However investigations by the inventors revealed that, since crystal defects such as dislocation induced by mechanical processing are likely to distribute along the cleavage plane, the GaP transparent semiconductor layer having the {110} surface on the side faces thereof tends to cause a large number of crystal defects in parallel with the side faces of the layer, raising a fear of adversely affecting manufacture of the devices against expectations. More specifically, the AlGaInP light emitting layer section and the GaP transparent semiconductor layer tend to produce therebetween mismatch-induced stress due to difference in the lattice constants, so that dicing along the {110} surface, which is a cleavage plane, is likely to cause laminar cracks along the cleavage plane (and consequently the chip edge) under mismatch-inducing stress, and to cause failures such as chipping of the chip edge or the like.
It is therefore a subject of this invention to provide a method of fabricating a light emitting device having an AlGaInP light emitting layer section and a GaP transparent semiconductor layer, less causative of failures such as edge chipping during dicing, and a light emitting device obtainable by the this method.